1. Technical Field
The present invention relates to a coincidence determination method and an apparatus of a PET device. Especially, the present invention relates to a coincidence determination method and an apparatus of a PET device that are preferred to be used for a PET device that employs a scintillator containing an Lu radioactive isotope as a radiation detector, and that remove background noise due to intrinsic radioactivity.
2. Related Art
A coincidence determination method (see H. M. Dent, W. F. Jones, and M. E. Casey, “A real time digital coincidence processor for positron emission tomography”, IEEE Trans. Nucl. Sci. Vol. 33, 556-559, 1986, and D. F. Newport, H. M. Dent, M. E. Casey, and D. W. Bouldin, “Coincidence Detection and Selection in Positron Emission Tomography Using VLSI”, IEEE Trans. Nucl. Sci. Vol. 36, 1052-1055, 1989) used in a PET device is, as illustrated in FIG. 1, a detection method that estimate a pair of annihilation radiations 14a and 14b detected within an extremely short period of approximately several nanoseconds as true coincidence generated from the same positron nuclide 12. In FIG. 1, 10 denotes an inspection target such as a patient; 20 denotes a detector ring (hereinafter, also referred to simply as a ring) where a plurality of radiation detectors (hereinafter, also referred to simply as detectors) 22 that constitutes the PET device are arranged, for example, on its circumference; 24 denotes circuits that detect detected positions and time information of radiations in the respective detectors 22; 26 denotes a coincidence circuit that determines a coincidence to be present if a difference in detection time between the plurality of detectors 22 falls within a predetermined coincidence time width (hereinafter, also referred to as coincidence time window); and 28 denotes a data collecting unit that collects and stores coincidence data.
The coincidence time window for determining a positron nuclide is determined by the timing resolution and the size of the field of view of the PET device. At present, PET devices with an improved timing resolution of around 500 picoseconds have been developed. The coincidence time window is also limited by the locations of positron nuclides and the ring diameter of the detector ring. A coincidence time window equal to or less than around four nanoseconds will confine the imaging field of view of existing clinical PET devices.
The coincidence method performs a positron nuclide determination in a finite time. A random coincidence as exemplarily illustrated in FIG. 2A that simultaneously detects annihilation radiations from different positron nuclides, and a scatter coincidence as exemplarily illustrated in FIG. 2B occur in addition to a true coincidence illustrated in FIG. 1.
Regarding this coincidence determination method, the applicant proposes several methods (see WO2011/117990 and WO2011/125181).
On the other hand, in a general PET device, since high timing resolution and similar performance results in good overall performance balance, a scintillator (LSO, LYSO, or LGSO) containing Lu radioactive isotope is employed. The Lu radioactive isotope of Lu-176 undergoes, as illustrated in FIG. 3, beta decay and emits a beta particle at a mean energy of 420 keV, and then emits three gamma rays at 307 keV, 202 keV, and 88 keV and becomes Hf-176. Accordingly, the PET device that employs the scintillator containing the Lu radioactive isotope contains background noise (which is referred to as intrinsic radioactivity) caused by Lu-176.
However, in measurement for a general clinical PET, this noise due to intrinsic radioactivity is negligible (see S. Yamamoto, H. Horii, M. Hurutani, et al., Ann. Nucl. Med., Vol. 19, 109-114, 2005). On the other hand, when imaging a positron nuclide at a low activity level, degradation in image quality due to artifact caused by a noise component of Lu-176 has been reported (A. L. Goertzen, J. Y. Suk, C. J. Tompson, J. Nucl. Med., Vol. 48, 1692-1698, 2007).
In particle radiotherapy, monitoring whether irradiation is correctly performed as planned by applying a principle of PET, what is called in-beam PET, also has noise due to intrinsic radioactivity as an obstacle for imaging the positron nuclide at the low activity level generated by irradiation with heavy particles.
On the other hand, in a PET device with very high timing resolution, information on time-of-flight (hereinafter, abbreviated as TOF) of a pair of annihilation radiations is used for limiting a position on a line of response (LOR) to improve sensitivity of the device. Such TOF-PET device has been developed.
Nowadays, a TOF-PET device using LSO has been put into practical use. In this TOF-PET device, the LOR is locally written. Thus, this not only reduces noise diffusion to improve SIN ratio of an image, but also contributes to reduction in random coincidence. This effect has been reported (see M. Conti, IEEE Trans. Nucl. Sci., vol. 53, 1188-1193, 2006, and J. A. Kimdon, J. Qi, and W. W. Moses, Nuclear Science Symposium Conference Record, 2003 IEEE, vol. 4, 2571-2573, 2003).
This TOF-PET device intrinsically removes the random coincidence, and has an effect on removal of background noise of Lu-176. This TOF-PET device has considerably high performance while the TOF-PET device is expensive. At this time, this device is not implemented to every PET device.
A general method for removing background noise of the scintillator containing Lu-176 is to narrow an energy window so as to focus only on a photopeak of the positron nuclide (see Watson CC, et al., 2004. J. Nucl. Med. 45(5):822-826, and S. Yamamoto, H. Horii, M. Hurutani, et al., Ann. Nucl. Med., Vol. 19, 109-114, 2005). Using sufficiently narrow energy window in the existing PET device does not have any problem of influence of background noise in clinical use.
However, even if the energy window is narrowed, it is not possible to remove a background component that causes a random coincidence. Accordingly, a problem arises in that this degrades image quality in measurement for radioactive concentration at a low activity level.
The present invention has been made to solve the existing problem, and it is an object of the present invention to effectively reduce background noise due to intrinsic radioactivity in the case where a scintillator containing the Lu radioactive isotope is used.
Most beta particles from Lu-176 are assumed to be detected inside the detector. As exemplarily illustrated in FIG. 4A, a case where the beta particle and the gamma ray cause a coincidence (Intrinsic True: IT), and as exemplarily illustrated in FIG. 4B, a case where the beta particles cause a random coincidence (Intrinsic Random: IR) are assumed. A ratio of count of these noise components significantly depends on a size of the PET device, the energy window, and the coincidence time window.
While IR is considered to be mainly counted as a coincidence of the beta particles, IR always accompanies the gamma rays. These gamma rays are not detected under an optimal energy window while the gamma rays are detected as Multiple Coincidences (MC) under a very wide energy window. That is, Lu-176 emits, as illustrated in FIG. 3, three gamma rays and a beta particle. Thus, in the case where radiation from Lu-176 in different positions of occurrence cause a random coincidence, a lower limit value of the energy window is decreased so as to detect a gamma ray from Lu-176. This allows high probability of multiple coincidences. On the other hand, in the PET measurement at a low activity level, multiple coincidences caused by the inspection target hardly occurs.
Current PET devices usually have implemented list mode data collection that collects a coincidence event in time order. To list mode data, not only an address of LOR but also TOF information, energy information, and similar information can be added. Accordingly, data reprocessing such as changing the energy window after collection is possible.
Therefore, also in an LSO-PET device without TOF detectability, after measurement with a very wide energy window, multiple coincidences are removed. Subsequently, the event is removed again using the existing energy window, which is considered to reduce IR.